Assessment of vascular (i.e., artery and vein) conditions is crucial to the diagnosis of many serious, and often life threatening, pathologies. For example, vascular occlusions, which commonly take the form of atherosclerotic vessel disease, can reduce or eliminate the flow of blood to critical organs within a body, thereby causing illness, disability and/or death. In particular, one or both of the carotid arteries supplying blood to the brain may become blocked and cause what is commonly referred to as a stroke. As is well known, a stroke often results in lasting disability and can result in death. Further, portions of the aorta, which is a major artery within the body that conveys blood from the heart to organs throughout the body, may become diseased, particularly in the abdominal region. A diseased aorta can cause severe pain and may eventually form an aneurysm that ruptures and causes death. Still further, femoral and popliteal arteries may become blocked. Typically, blockages in the femoral and popliteal arteries form near the groin and legs and cause weakness in the legs. If these blockages in the femoral and popliteal arteries are not diagnosed and treated in a timely manner, amputation of one or both legs may be required. Still further, the renal arteries, which convey blood from the abdominal aorta to the kidneys, may become blocked, thereby causing hypertension, kidney failure and ultimately death.
One particularly problematic vascular condition occurs in patients that have chronic renal (i.e., kidney related) failure. As is well known, renal dialysis and vascular access to carry out the dialysis are critical aspects of managing chronic renal failure. In fact, without some form of dialysis, the more than 120,000 people in the United States with chronic renal failure would rapidly succumb to their disease.
Hemodialysis is the most commonly employed renal dialysis technique. To effectively carry out hemodialysis, large needles must be inserted into large blood vessels so that substantial quantities of blood can be processed by the dialysis equipment in a relatively short period of time. Additionally, because hemodialysis must typically be performed several times each week for months or possibly years, arterial-venous (AV) access shunts are needed to provide long-term vascular access. As is well known, an AV shunt is disposed between an artery and a vein, usually located in the patient's forearm, to enable blood to flow directly between the artery and the vein. The AV shunt provides the large blood vessel that is needed to accommodate the relatively large dialysis needle and bypasses high resistance vessels such as arterioles and capillaries to facilitate the high blood flow rates needed to accomplish efficient hemodialysis. In practice, an AV shunt may be created by surgically placing a graft, which is typically either made of an artificial material or scavenged from a vessel in another location of the body such as, for example, the leg. Alternatively, the AV shunt may be a fistula, which is created using direct anastomosis of an artery and a vein.
While the above-described AV shunts initially function properly, these AV shunts typically become clogged (with blood clots, for example, which may be caused by hyperplasia) over time. Furthermore, although there are a variety of techniques which can be used to treat (i.e., eliminate or reduce) vascular blockages such as, for example, angioplasty, early diagnosis of the blockage is needed to minimize risk to the patient and to maximize the likelihood that treatment will successfully reduce the blockage to a safe level or eliminate the blockage completely.
Further, due to the limited number of areas on a patient's body which may be used for hemodialysis access, it is crucial that access sites are preserved as long as possible. AV grafts account for about 75% of dialysis access devices and over one-half of these AV grafts require angioplastic or other salvage intervention within the first year. With AV fistulas, on the other hand, about 30% are unusable due to a failure to mature. Additionally, of the AV fistulas that successfully mature, about 15% require radiologic or surgical revision within one year.
At present, physical examination is a commonly used technique for assessment of vascular patency. Physical examination techniques are particularly useful for detecting very low blood flow which, in the case of an AV shunt, is indicative of an impending shunt failure. As is well known, vascular flux pulse and thrill (i.e., a vascular murmur) and auscultation may be used to assess vascular patency. Generally speaking, a palpable murmur or thrill is indicative of a reasonable blood flow (e.g., greater than about 450 milliliters/minute), a sharp pulse indicates lower blood flows and an increased bruit (i.e., an abnormal sound) suggests a vascular stricture or stenosis.
Longitudinal monitoring of blood flow is another well-known technique for detecting significant vascular stenosis. However, longitudinal monitoring techniques are less desirable in practice because these techniques require standardization of tubing size, needle size and other hemodialysis equipment. Still further, duplex color Doppler ultrasound flow studies, dilution and magnetic resonance imaging are other well-known techniques for assessing vascular patency. Unfortunately, these well-known techniques are relatively expensive, of limited availability, and the results obtained with these techniques depend heavily on the skill level of the observer.